This invention relates generally to systems for gas phase chemical processes, and more particularly, to a system coupling a periodic separation process with chemical reaction.
The use of pressure swing adsorption processes to effect separation of gas mixtures of the type having selectably adsorbable components is well known. In known systems, the discharge product gas is not continuous, and therefore, a plurality of adsorbent beds are provided in parallel with one another to achieve a measure of continuity in the product output flow. In the separation process of each adsorption bed at least one selectable component of the feed gas mixture is adsorbed so that the gas discharged at the other end of the adsorption bed is the component-depleted product gas. Generally, such adsorption occurs at the highest pressure of the process which is generally the input feed pressure. This high pressure portion of the separation process cycle is followed by a depressurization portion of the cycle wherein the gas within the adsorption bed is reversed in its direction of flow and released at the inlet end of the adsorption bed. The gas which is thus exhausted is rich with the desorbate, which corresponds to the component of the feed gas which had been adsorbed and is released upon reduction in the pressure. In certain known systems, the depressurized exhaust portion of the cycle is followed by introduction of a purge gas at the product outlet end of the adsorption. A new cycle is commenced with the introduction once again of pressurized feed gas after purging has been completed.
The prior art has recognized that multiple adsorption bed systems are plagued with various significant disadvantages. For example, in addition to the requirement of multiple adsorption beds, substantial complexity and expense is required in the interconnecting piping and valving system. Additionally, such multiple adsorption bed systems require long cycle periods. Moreover, such systems achieve only unacceptably low utilization, or productivity, of the adsorbent.
The prior art has attempted to mitigate some of the problems associated with multiple bed adsorption systems by providing the rapid pressure swing adsorption process (RPSA). A significant difference between the pressure swing adsorption system (PSA) as described hereinabove with multiple adsorption beds and RPSA, is that RPSA utilizes a higher flow resistance in the adsorbent bed as a result of packing the bed with small adsorbent particles.
PSA, in contrast, minimizes the flow resistance and thereby operates with reduced pressure drop across the adsorption bed. The flow of feed gas at a first pressure, which is typically the highest pressure applied to the adsorbent bed during the process cycle, continues for a predetermined portion of the overall process cycle; and during that time, a product gas is issued at the other end of the adsorbent bed. The product gas corresponds to the feed gas which has been depleted by a gas component which has been adsorbed in the adsorbent bed. During a second portion of a product cycle, the inletting of pressurized feed gas is discontinued and an exhaust valve is opened. The exhaust valve is located at the feed end of the adsorbent bed. This results in a significant decrease in the gas pressure at the adsorbent bed, and therefore the adsorbed component gas is released. This phenomenon, which is now well known after its discovery by one of the inventors herein, essentially involves the correlation that the capacity of the adsorbent bed increases with pressure or temperature. Thus, desorption occurs when the pressure is reduced. During this period of pressure reduction, the component-depleted gas flows in the opposite direction from the feed gas flow, toward the exhaust valve. This reverse-flowing gas is therefore rich in the adsorbed component gas, and may itself be a usable gas.
The foregoing is but one of the many operations which have been studied in the prior art under controlled cyclic operation. It is noteworthy that the various operations of such cyclic systems cannot be analyzed in a steady state context. The pulsed operations utilize repetitive parameter changes, such that the system behavior remains transient notwithstanding a cyclic steady state. It is now known, however, that cyclic operation results in improved efficiency and increases throughput for such processes as absorption, extraction, crystallization, ion exchange, reaction, distillation, adsorption, and particle separation. These results are particularly apparent in mass production cyclic reactors including catalytic converters and gas heating furnaces.
The prior art has generally concentrated on either separation or reaction processes. Several approaches have been employed to achieve cyclic operations, the most common being the cycling of a thermodynamic variable which effects separation. Such thermodynamic variables include temperature, concentration, electric fields, pH, and pressure. Separation processes of this nature depend upon the cyclic variation of the distribution coefficient for a solute between phases. One phase acts as a capacitor, alternately storing and losing solute to the other phase involved. In some cyclic separation schemes, such as parametric pumping and pressure swing adsorption, the process includes a flow reversal which is coupled to a change in a thermodynamic variable.
Reactor performance is known to be altered by cyclic operation for any nonlinear process. For heterogeneous catalytic gas phase reactors, surface rate processes, such as adsorption diffusion, surface reaction, and desorption, are influenced by periodic operation, even to the point of changing the dominating reaction mechanism. Generally, the feed concentration and the reactor temperature are the variables which are cycled.
It is an object of this invention to provide a periodic chemical processing system wherein both, reaction and separation, are achieved in a single bed.
It is another object of this invention to provide a system wherein the cyclic separation process of pressure swing adsorption is combined with a cyclic reaction process.
It is a further object of this invention to provide a periodic chemical processing system wherein the cycling of feed pressure, temperature and/or concentration is combined with the internal distribution of an adsorbent for separation and a catalyst to facilitate reaction.
It is also an object of this invention to provide a periodic chemical processing system wherein reaction of components in the system varies the adsorption behavior of the gaseous components to increase adsorption of a selectable one, and hence, to create better selectivity than obtainable with known systems.
It is additionally an object of this invention to provide a system which operates substantially in accordance with a reaction-rate limited model given herein so that the effect of variations in process parameters can be predicted.
It is still another object of this invention to provide a periodic chemical processing system wherein the separation mode can be altered to permit control of which exit stream will contain the desired component.
It is an additional object of this invention to provide a periodic chemical processing system wherein costs are reduced by elimination of the need of multiple separators.
It is still a further object of the invention to combine a continuous cyclic separation process with a periodically operated reactor.
It is a yet further object of this invention to provide a periodically-operated heterogeneous catalytic reactor-adsorber.